1. Field of the Invention
The present invention relates generally to an automotive power unit including an automatic transmission and more specifically to a diagnostic arrangement for monitoring the operation of a sensor or sensors utilized therein.
2. Description of the Prior Art
JP-A-62-62047 discloses an automotive transmission which is electronically controlled in a manner to minimize shift shock. Data such as engine and vehicle speed are input to a A/T control unit which processes the same and generates the various necessary shift and line pressure control signals. Further, this type of transmission uses a conventional type torque converter wherein the engine torque which is input thereto, is transmitted to the transmission input shaft by the turbine. Accordingly, as the transmission input speed can vary considerably from the engine speed during various modes of operation, in order to control the transmission in the required manner it is deemed advantageous to know the rotational speed of the transmission input shaft and to be able to compare this value with the rotational speed of the output shaft.
The sensors which are used in this arrangement are provided with diagnostic and fail-safe back-up arrangements in order to obviate totally erroneous operation in the event that one or more of the sensors should suddenly begin to malfunction and supply totally erroneous data to the control unit.
For example, in the case of the turbine sensor, when the vehicle speed is above a predetermined level and the turbine sensor output indicates an abnormally low turbine rotational speed, the turbine sensor can be diagnosed as being out of order. Alternatively, in the case of the engine speed sensor indicating a high rotational speed while the turbine speed is indicated as being below a predetermined level it is possible to predict that the turbine sensor is not functioning properly.
However, with this type of diagnostic arrangement it is possible in the case the transmission is conditioned to produce neutral (such as in response to a D-N select for example) for the engine speed to be quite high (e.g. engine racing) while the turbine speed is quite low or even zero. This opens the door to the possibility of an erroneous malfunction indication. Further, in the case of a vehicle coasting and the engine speed lowers to the point where the output pressure of the oil pump drops off and permits the power train to assume a pseudo neutral condition wherein the reverse flow of torque from the driving wheels back through the transmission to the turbine does not occur, it is possible for the turbine speed to quite low while the vehicle speed is high, and again induce the situation wherein an erroneous indication that the turbine sensor is out of order, may be produced.
A further example of a situation wherein erroneous sensor diagnosis is possible occurs in the case wherein the vehicle stalls--the engine speed is high but the vehicle progress is so inhibited (e.g. extremely steep incline) that the speed is very low. Under these conditions the turbine speed will be sensed as being too low for the instant engine speed and thus the diagnostic system is apt to generates an erroneous sensor malfunction indication.